Focal Depth Classification and Seismotectonics

Core Idea

Earthquakes are classified as shallow (<70 km), intermediate (70–300 km), or deep (>300 km) based on focal depth. Shallow earthquakes occur at all plate boundaries; intermediate and deep earthquakes occur primarily in subduction zones where cold, sinking lithosphere remains brittle at depths where surface rocks would deform plastically. Focal depth patterns reveal lithospheric structure and plate convergence geometry.

Explainer

From your work on earthquake location and hypocenter determination, you know that seismologists can pinpoint not just where an earthquake occurs on the surface but how deep within the Earth it originates. That depth — the focal depth — turns out to be one of the most informative measurements in seismology, because it reveals which tectonic process is generating the earthquake.

Shallow earthquakes occur at depths less than 70 km. These are by far the most common and occur at every type of plate boundary: transform faults, spreading ridges, and subduction zones alike. They happen within the brittle upper lithosphere, where rocks respond to stress by fracturing — the same brittle failure mechanism you would expect from your understanding of plate tectonics. Most destructive earthquakes are shallow, because the energy release is close to the surface.

Intermediate-depth earthquakes (70–300 km) and deep earthquakes (300–700 km) are far more restricted in their geography. They occur almost exclusively in subduction zones, where one tectonic plate descends beneath another. This is initially puzzling: at those depths, the surrounding mantle is hot enough that rocks should flow plastically rather than snap. The key insight is that the subducting slab is old, cold oceanic lithosphere. It sinks faster than it can warm up, remaining hundreds of degrees cooler than the surrounding mantle at the same depth. This thermal anomaly preserves brittle behavior — or enables alternative failure mechanisms like dehydration embrittlement, where minerals in the slab release water at specific pressure-temperature conditions, triggering sudden fracture.

The spatial pattern of these deep earthquakes is strikingly systematic. In the 1930s and 1940s, Hugo Benioff and Kiyoo Wadati independently noticed that earthquake foci in subduction zones define an inclined plane dipping from the trench into the mantle — now called the Wadati-Benioff zone. The angle and extent of this seismic zone maps the geometry of the descending slab. Steeply dipping slabs (like beneath the Mariana Trench) produce earthquakes down to nearly 700 km, while shallowly dipping slabs (like beneath parts of South America) spread their seismicity over a wider horizontal area. Below about 700 km, earthquakes cease entirely — the slab has either warmed enough to deform plastically, or it has reached the transition zone where mineral phase changes absorb the strain. Focal depth classification thus transforms earthquake catalogs into three-dimensional maps of lithospheric structure.